In some printers and other hard copy devices, the composition of the ink used is continually monitored and adjusted and/or replenished in use. For example, in a liquid-ink electrostatic printer, the ink solution may consist of a blend of ink concentrate and conductivity agent in an oil. In use of such an electrostatic printer, the ink solution is replenished by a solution often consisting of fresh oil, ink concentrate, and conductivity agent as needed. The solution is constantly mixed, for example, by being circulated by a pump, to produce a uniform composition.
If the level of ink concentrate or conductivity agent becomes too high, or if the ink solution becomes contaminated, or if it is desired to substitute a different ink solution, it has in the past been generally necessary to drain the relevant ink tank on the printer, clean the ink tank and pipe-work, refill the ink tank with clean oil, and build up the ink solution to a usable concentration of ink concentrate and conductivity agent. Both the cleaning and the rebuilding of the ink solution take considerable time, during which the printer is unable to print. Because the ink solution generally requires continual mixing, if the printer is powered down for an extended length of time the ink concentrate and oil content separates, and must be cleaned out either at shutdown or at startup, and the ink solution must be rebuilt before the printer can be brought back into operation.